Grasshopper and the Falcon 9R Dev were the two rockets of spaceX which landed back successfully. Those were limited altitude test flight, this one were the expended first stage of a revenue flight. This rocket went higher, and faster before separation and performing a boost back burn. Might seem as just a quantitative difference, but the different aerodynamical regimes (vacum, hypersonic, supersonic, transonic) provides a real challenge.

It's also worth noting that the engines were lit 100% during those previous tests, while they have to be relight during this one. That comes with it's own set of complications of course.

see more: http://en.wikipedia.org/wiki/Grasshopper_%28rocket%29 and http://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_d...